The invention relates to a method for starting a fuel cell system having a fuel cell stack, a compressor for supplying a working medium to the fuel cell stack, and an electric motor for driving the compressor. The invention also relates to a fuel cell system having a fuel cell stack, a compressor for supplying a working medium to the fuel cell stack, and an electric motor for driving the compressor.
German Patent DE 43 22 767 C2, corresponding to U.S. Pat. No. 5,794,732, discloses a fuel cell system and a method for starting a fuel cell system of the type mentioned initially. The fuel cell system described in that document is an air and hydrogen system. A compressor which can be powered by an electric motor is provided to compress the air in the known system. The electric motor is supplied from a mobile starter battery during a starting phase.
Since, in the prior art, the output voltage of the starter battery is different from the operating voltage of the fuel cell stack of the fuel cell system, which is present at the output of the fuel cell stack, a separate electric motor, which is constructed for the higher voltage of the fuel cell stack, is provided as well as the starter motor in the prior art. Such an apparatus is relatively complex since it requires two separate electric motors, which must be coupled to the compressor, occupying space and volume.
It is accordingly an object of the invention to provide a method for starting a fuel cell system and a fuel cell system, which overcome the herein afore-mentioned disadvantages of the heretofore-known methods and devices of this general type, which simplify operation of a compressor, which require less structural complexity and which occupy less space for the fuel cell system.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for starting a fuel cell system, which comprises providing a fuel cell stack; supplying a working medium to the fuel cell stack with a compressor; driving the compressor with an electric motor; initially feeding the electric motor with current from a starter battery; and then feeding the electric motor with current from the fuel cell stack.
The essence of the invention is to switch the power supply for the electric motor from the starter battery to the fuel cell stack and, if necessary, back again. The switching process takes place when the voltage at the output of the fuel cell stack reaches an adequate level. The electrical connection between the output of the starter battery and the motor input is interrupted, and the motor input is switched to the output voltage of the fuel cell stack.
In accordance with another mode of the invention, the fuel cell system is a mobile air and hydrogen system, in particular for a vehicle, such as a construction industry vehicle, a vehicle for use indoors, a bus or the like. The fuel cell system is supplied with air as the working medium during starting and operation, and the air is conveyed by the motor-driven compressor in order to ensure the necessary operating pressure and, if necessary, additional humidifying of the air. One suitable fuel cell system is a PEM fuel cell system with PEM fuel cells. In this case, PEM stands for Proton Exchange Membrane or Polymer Electrolyte Membrane.
In accordance with a further mode of the invention, during the starting phase of the fuel cell system, the electric motor of the compressor is advantageously supplied from the starter battery. During the starting phase, the fuel cell block does not yet develop any voltage, or does not yet develop the necessary voltage, in order to operate the electric motor.
In accordance with an added mode of the invention, a control device is provided for switching the power supply of the electric motor. The control device interrupts the electrical connection between the output of the starter battery and the motor input and switches over the motor input to the output voltage of the fuel cell stack when the output voltage of the fuel cell stack reaches an adequate level.
In accordance with an additional mode of the invention, the operating voltage of the electric motor advantageously corresponds to the voltage of the starter battery, and the control device advantageously includes a converter which regulates the output voltage of the fuel cell stack down to the operating voltage of the electric motor, or less. A current regulator is able to regulate the output voltage of the fuel cell stack down to the operating voltage of the direct-current motor and, in the process, also advantageously maintain this direct-current motor at a constant rotational speed.
The control device advantageously has two functions: firstly the selection and switching of the supplied operating voltage for the electric motor during the starting phase and during the operating phase of the system, and secondly the advantageous permanent regulation of the motor, which is important for optimized operation of the fuel cell stack, since the compressor is used to provide reproducible and advantageously optimized power levels.
In accordance with yet another mode of the invention, the switching from the starter battery to the output of the fuel cell stack advantageously takes place when the output voltage of the fuel cell stack is higher than the voltage of the starter battery. That is because, in this case, the converter just has to regulate the output voltage level of the fuel cell stack downward.
In accordance with yet a further mode of the invention, the output voltage of the fuel cell stack is advantageously approximately twice as great as the voltage of the starter battery. This refinement of the invention allows the electrical and electronic components of the converter to still have small dimensions for a mobile system.
In accordance with yet an added mode of the invention, the converter is a direct-current controller (DC/DC controller) and the electric motor is a direct-current motor.
In accordance with an alternative mode of the invention, the electric motor is a synchronous motor or an asynchronous motor and the control device includes a converter which is in the form of an inverter, for example a DC/three-phase AC inverter. Furthermore, the electric motor has two separate winding systems, with the first winding system being constructed for the voltage of the starter battery, and the second winding system being constructed for the higher voltage of the fuel cell stack.
In accordance with yet an additional mode of the invention, the control device also provides the motor regulation and the switching and selection of the two voltage sources. The first winding, which is constructed for the voltage of the starter battery, can be isolated from the voltage sources through disconnection elements. The disconnection elements can be controlled by the switching apparatus, in the form of a DC/three-phase AC controller. The second winding system, which is constructed for the operating voltage of the fuel cell stack, is continuously connected to the controller output.
Current is expediently drawn from the starter battery only until the voltage level at the output of the fuel cell stack has reached a changeover value, which can be preset. This is greater than the voltage level of the starter battery.
In accordance with again another mode of the invention, in order to protect the electrical components and the motor, the first winding system of the electric motor is expediently disconnected at the zero crossing.
With the objects of the invention in view, there is also provided a fuel cell system, comprising a starter battery; a fuel cell stack; a compressor for supplying a working medium to the fuel cell stack; and an electric motor for driving the compressor, the electric motor to be selectively operated with current from the starter battery and with current from the fuel cell stack.
In accordance with another feature of the invention, the fuel cell system is a mobile fuel cell system which can be operated with air and hydrogen, for powering vehicles. The fuel cell system includes at least one fuel cell stack, which in turn includes a number of fuel cells. The fuel cells are supplied, for example, with hydrogen and compressed air through a respective inlet. An air compressor is connected to the inlet of the fuel cells and has a motor drive with an electric motor which can be supplied from a starter battery.
In accordance with a further feature of the invention, the motor is connected to the power sources through an electronic control device. The control device has a converter having an intermediate circuit, that is, for example, a direct-current controller or an inverter, depending on whether the electric motor of the compressor is a direct-current motor, an alternating-current motor, or a three-phase motor.
In accordance with an added feature of the invention, apart from its control function, the control device advantageously also has a switching function and thus has a switching apparatus, which is connected electrically upstream of the input of the converter.
In accordance with an additional feature of the invention, the output of the converter is expediently permanently connected to the input of the electric motor.
In accordance with again another feature of the invention, if the control device is in the form of an inverter, for example a DC/three-phase AC controller, both the output of the starter battery and the output of the fuel cell stack can be permanently connected through decoupling diodes to the input of the DC/three-phase AC controller.
In accordance with again a further feature of the invention, the electric motor (which is in the form of a three-phase motor) of the compressor has two separate winding systems. In this case, the first winding system is constructed to be supplied with the low voltage of the starter battery, and the second winding system is constructed to be supplied with a higher voltage, namely the voltage of the fuel cell stack.
In accordance with again an added feature of the invention, the first electric motor winding system, which is constructed for the lower voltage, can be expediently disconnected as soon as a changeover voltage level, which could be dangerous to the sensitive first winding system, is reached.
In accordance with a concomitant feature of the invention, the actual disconnection apparatus is advantageously integrated in the electric motor, or the compressor motor.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for starting a fuel cell system and a fuel cell system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.